Devices and methods for reduction of NOx emissions from lean burn engines

ABSTRACT

The invention provides devices and methods for generating H 2  and CO in an O 2  containing gas stream. The invention also provides devices and methods for removal of NO X  from an O 2  containing gas stream, particularly the oxygen-rich exhaust stream from a lean-burning engine, such as a diesel engine. The invention includes a fuel processor that efficiently converts added hydrocarbon fuel to a reducing mixture of H 2  and CO. The added fuel may be a portion of the onboard fuel on a vehicle. The H 2  and CO are incorporated into the exhaust stream and reacted over a selective lean NO X  catalyst to convert NO X  to N 2 . thereby providing an efficient means of NO X  emission control.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application60/426,604, filed on Nov. 15, 2002.

FIELD OF THE INVENTION

This invention relates to reduction of NO_(X) emissions from acombustion process. The invention also relates to removal of NO_(X) fromthe exhaust from an internal combustion engine, particularly a lean burninternal combustion engine, such as a diesel engine. The invention alsorelates to fuel processing, whereby a hydrocarbon fuel is converted intoa reducing gas mixture containing H₂ and CO, and use of the H₂ and CO toreduce NO_(X) in an oxygen-rich exhaust stream.

BACKGROUND OF THE INVENTION

Emissions regulations are continually being tightened to improve airquality in many locations around the world. Over the past 30 years, theregulations relating to spark ignited engines have been tightened andthe allowable emissions substantially reduced. These engines operate ator near a stoichiometric air/fuel ratio and as a result three-waycatalyst technology has been developed to control the emissions ofcarbon monoxide (CO), unburned hydrocarbons (UHC), and nitrogen oxides(NO_(X)), including both NO and NO₂. Three-way catalyst technology isnot applicable to lean burn engines because the large excess of oxygenin the exhaust mixture impedes the reduction of NO_(X). This isparticularly a problem with diesel engines or compression ignitionengines which have very high emissions of NO_(X) and particulate matter(PM). Coupled with this is the drive within the U.S. and much of theworld for increased fuel efficiency. Diesel engines are very efficientand therefore a desirable power source for vehicles. However, the highemissions must be reduced to comply with statutory requirements. Toreach the emissions levels required for gasoline spark ignited engines,the emissions from a modern diesel engine must be reduced by a factor of10 to 50, depending on the engine.

Lean-burn engines include both spark-ignition (SI) andcompression-ignition (CI) engines. Lean-burn SI engines offer 20-25%greater fuel economy and CI engines offer 50% and sometimes higher fueleconomy than equivalent conventional SI engines. CI engines are widelyused in heavy-duty vehicles, and their use in light-duty vehicles issmall but expected to grow. They are also widely used in stationaryapplications, such as electric power generators.

Current automotive emission control technology is based largely oncatalytic converters with three-way catalysts (TWCs). This technology ishighly effective for ordinary gasoline engines that operate at nearlystoichiometric air/fuel ratios. However, as discussed above, it isincompatible with lean-burn engines due to the excess of oxygen in theexhaust. This incompatibility is a major limitation of both lean-burnengines and TWC-based emission control technology. In the case of dieselengines, the emission control system must remove NO_(X) and PM from anexhaust stream containing about 6-15% oxygen.

Many different technologies have been investigated for NO_(X) removalfrom lean-burn engine exhaust. One successful technology has been theselective reduction of NO_(X) with ammonia (NH₃) as a reducing agent.Ammonia is added to the exhaust stream in an amount proportionate to theamount of NO_(X). The exhaust stream containing NO_(X) and NH₃ is thenpassed over a catalyst upon which the NO_(X) and NH₃ selectively reactto produce N₂. This technology, referred to as Selective CatalyticReduction (SCR), is widely used in gas turbines and large boilers andfurnaces, and is capable of achieving very high NO_(X) conversion to N₂.However, one disadvantage of this technology is that it requires asource of NH₃ which can be either liquid NH₃ stored under high pressureor an aqueous solution of urea which decomposes prior to or on the SCRcatalyst to produce ammonia. In general NH₃—SCR technology is limited tolarge stationary applications, due to its cost and the need for a sourceof NH₃. In addition, the addition of NH₃ must be carefully controlled toachieve a desired NH₃/NO_(X) ratio, to prevent excess NH₃ from beingexhausted to the atmosphere and adding to the level of air pollutants.Another disadvantage is the need to develop the rather costlyinfrastructure to supply ammonia or urea to vehicles using thistechnology. For these reasons, this technology is not the preferredapproach to NO_(X) control on vehicles or very small systems that may belocated in populated areas.

Another technology that has been explored for NO_(X) abatement fromlean-burning mobile sources is a NO_(X) storage and reduction (NSR)system, as described in Society of Automotive Engineers papersSAE-950809 and SAE-962051, and in U.S. Pat. No. 6,161,378. The NSRsystem has an adsorbent-catalyst unit situated in the exhaust system andthrough which the exhaust stream flows. This catalyst unit provides twofunctions: reversible NO_(X) storage or trapping, and NO_(X) reduction.During normal engine operation, while the exhaust gas flows through thesystem, NO_(X) is adsorbed onto the adsorbent in the presence of excessoxygen during the Adsorption Cycle. As the adsorbent component becomessaturated with NO_(X), the adsorption becomes less complete and theNO_(X) exiting the NO_(X) trap begins to increase. At this point, thecomposition of the exhaust stream is changed from an oxidizing to areducing state. This requires reduction of the oxygen level to zero andintroduction of a reducing agent. In the reducing environment, theNO_(X) is desorbed from the adsorbent and then reduced to nitrogen bythe catalytic components that are incorporated into theadsorbent-catalyst unit. This reaction is generally very quick. Thus,the reduction part of the cycle can be very short, but must besufficiently long to regenerate a significant fraction of the NO_(X)adsorption capacity. The exhaust composition is then reverted to normaloxidizing conditions, and the cycle is repeated. There are severaldisadvantages to this technology. One problem is that converting theexhaust to reducing conditions is difficult to achieve for a lean-burnengine such as a diesel engine designed to run with 8 to 15% O₂ in theexhaust stream. Another problem is that the adsorbent-catalystcomponents that have been investigated form very stable sulfates,resulting in poisoning of the catalyst by sulfur in the fuel.Regeneration of the catalyst to remove the sulfur is very difficult andresults in degradation of the catalyst performance.

A promising approach to NO_(X) removal from exhaust streams containingexcess O₂ is selective catalytic reduction of the NO_(X) with a reducingagent such as CO or an added hydrocarbon, using a catalyst called aselective lean NO_(X) catalyst (“lean NO_(X) catalyst”) Such catalystshave been extensively investigated over the last 20 years (see, forexample, Shelef (1995) Chem. Rev. 95:209-225, and U.S. Pat. No.5,968,463). Previously, hydrocarbons have been used as the reducingagent, with the rationale that this component would be available fromthe engine fuel. In general, in engine tests using a lean NO_(X)catalyst, when a reactive hydrocarbon is used as the sole reducing agentor fuel is injected into a diesel engine in such a manner as to producereactive hydrocarbon species, the level of NO_(X) control is low, in therange of 20-50%.

Hydrogen has also been found to be a good reducing agent for theselective reduction of NO_(X) to N₂. For example, Costa, et al. (2001)J. Catalysis 197:350-364, report high activity of H₂ as a reducing agentfor catalytic reduction of NO_(X) in the presence of excess O₂ at lowtemperatures (150-250° C.), with good utilization of the H₂. EP1,094,206A2 also describes beneficial results associated with additionof H₂ to a hydrocarbon reducing agent in a lean NO_(X) catalyst system,resulting in greater than 95% NO_(X) removal in engine dynometertesting. H₂/CO mixtures have also been found to be good reducing agentsin such systems.

Although H₂ and H₂/CO mixtures are good reducing agents for continuousremoval of NO_(X) from an O₂-containing exhaust stream, present methodsfor delivering these reducing agents for use in a small mobile systemsuch as a vehicle are cumbersome and/or expensive. Hydrogen is difficultto store and H₂ refueling stations are currently not available. On-boardmanufacturing of H₂ or H₂/CO mixtures from diesel fuel is possible, butdifficult and costly.

An example of a system for on-board generation of a reducing agent forNO_(X) reduction may be found in WO 01/34950, which describes a partialoxidation system that uses air and the on-board hydrocarbon fuel togenerate a reducing mixture that is added to the exhaust stream. Theexhaust stream, which contains NO_(X), and the added reducing agent arethen reacted over a catalyst that reduces the NO_(X) in the presence ofexcess O₂. A disadvantage with such a device is that it may be difficultto operate for an extended period of time due to formation of coke,which ultimately poisons the catalyst. Also, this system produces lowmolecular weight hydrocarbons, which are less effective reducing agentsfor NO_(X) than H₂. Another system has been described in U.S. Pat. No.6,176,078 that involves use of a plasmatron to produce low molecularweight hydrocarbons and H₂ from hydrocarbon fuel. Disadvantages withthis system include high energy cost, cost of the system includingelectronics for the plasma generator and durability issues. U.S. Pat.No. 5,441,401 and EP 0,537,968A1 describe use of a separate H₂ generatorwith a separate air and water intake. Since the water is vaporized andpasses through the catalyst, it must be very pure. This would requireseparate tanks, supply system and complexity. However, this system maybe difficult to implement and too complex for NO_(X) removal in mobilesystems such as a vehicle. Another well-known technology includes anautothermal reformer (ATR) with a heat exchanger and water feed pumps.However, such a system is difficult to scale down. In addition, theseprocesses that convert the liquid hydrocarbon fuel to H₂ and CO in aseparate reactor system can have a long start up time, from 1 to 30minutes. This would result in a long period of time during which noreducing agent is available for NOx reduction and vehicle NOx emissionslevels would remain unacceptably high.

Both O₂ and H₂O may be used to convert a hydrocarbon fuel such as dieselfuel to H₂ and CO, through reactions such as partial oxidation and steamreforming in the presence of an appropriate catalyst. One approach thathas previously been used for processing a hydrocarbon fuel to produce H₂and CO is to add the fuel continuously to a gas stream upstream of acatalyst, which then converts the fuel to H₂ and CO when thefuel-containing gas stream contacts the catalyst. However, thedisadvantage of continuous fuel addition is that the high level of O₂ inthe exhaust stream results in a very high temperature at afuel-to-oxygen ratio that is good for reforming the fuel to H₂ and CO.This is depicted schematically in FIG. 1. FIG. 1 depicts the reactortemperature over time at various equivalence ratios (Φ). As fuel isadded to an oxygen containing gas stream, for example a diesel exhaustcontaining 10% O₂, combustion of the fuel results in heat release and anincrease in temperature. Thus, at an equivalence ratio of 0.2, theexhaust gas would increase in temperature from about 250° C. to about500° C. At an equivalence ratio of 0.5, the temperature would be 820°C., and at an equivalence ratio of 1.0, the temperature would be 1230°C. As the equivalence ratio rises above 1, the temperature decreases dueto endothermic reforming reactions. Thus, at an equivalence ratio of 2,the temperature would be 1042° C., and at an equivalence ratio of 3, avery rich mixture, the temperature would be 845° C. Typical auto-thermalreformers, which regulate temperature isothermally by using combustion(an exothermic reaction) to supply the heat for steam reforming (anendothermic reaction), operate with an equivalence ratio in the range of3 to 4, with a high level of steam (at least 30%) to increase the steamto carbon ratio (S/C₁) to a value above 1 and dilute the concentrationof O₂. Addition of steam is necessary in such a system to prevent cokeformation (carbon deposition) on the catalyst. However, addition ofsteam is not desirable because this water must be carried on the vehicleas a feed for the fuel processing system and the water would have to bevery pure since typical impurities in tap water such as sodium, calcium,magnesium etc. are poisons for most reforming catalyst materials.

BRIEF SUMMARY OF THE INVENTION

The invention provides devices, methods, and compositions for productionof reductant from a fuel in an oxygen containing gas stream, andreduction of NO_(X) emissions in an oxygen containing exhaust stream.

In one aspect, the invention provides a device for producing reductant.The device includes a fuel processor and a catalytic zone having anoxidation catalyst and a reforming catalyst, wherein the fuel injectoris configured to inject fuel into at least a portion of an oxygencontaining gas stream upstream from the catalytic zone to provide richand lean zones in the gas stream when the gas stream flows through thecatalytic zone. In one embodiment, the device is configured such that asa rich zone in an oxygen containing gas stream flows through thecatalytic zone in a direction from the inlet to the outlet of thecatalytic zone, a portion of the fuel in the rich zone is oxidized onthe oxidation catalyst and at least a portion of the remaining fuel inthe rich zone is reformed on the reforming catalyst, thereby producing areducing gas stream. In a rich zone of the oxygen containing gas stream,a portion of the added hydrocarbon fuel is oxidized on the oxidationcatalyst to consume substantially all (i.e., greater than about 90%) ofthe oxygen and the remaining fuel is converted to reductant, e.g., H₂and CO when a hydrocarbon fuel is used, on the reforming catalyst,thereby producing a reducing gas stream. The device may further comprisea reservoir containing a hydrocarbon fuel, wherein the reservoir is influid communication with the fuel injector and wherein the reducing gasstream includes H₂ and CO.

In one embodiment, a lean zone does not include added fuel. In anotherembodiment, a lean zone includes some fuel at an equivalence ratio lessthan 1. In embodiments in which lean zones contain some fuel, preferablyessentially all of the added fuel in the lean zone is oxidized on theoxidation catalyst as the lean zone flows through the catalytic zone oris oxidized on a pre-oxidation catalyst as described below.

In some embodiments, the oxidation catalyst and the reforming catalystare the same composition. In other embodiments, the oxidation catalystand the reforming catalyst are different compositions.

Creation of rich and lean zones in the gas stream may be accomplished bydiscontinuous injection of fuel to form rich and lean regions in the gasstream. In one embodiment, the fuel injector is adapted to introduce thehydrocarbon fuel to an oxygen containing gas stream discontinuously toform alternating rich and lean zones. Often, the duration of fueladdition to form a rich zone and a lean zone includes a rich-leanperiod, wherein the rich-lean period repeats every 0.1 to 10 seconds,and wherein the rich portion of the rich-lean period extends over about10 to about 90% of the rich-lean period.

Alternatively, rich and lean zones may be created by injecting fuelessentially continuously, with rich and lean zones formed by movingeither the catalytic zone or the fuel injector relative to the flow ofthe gas stream, or by changing the spray angle of fuel injection suchthat a varying portion of the catalytic zone receives a gas stream withadded fuel. In one embodiment, the fuel injector is adapted to introducethe fuel to a portion of an oxygen containing gas stream essentiallycontinuously to form a rich zone, and the device is configured such thatthe portion of the catalytic zone through which the rich zone flowsvaries over time.

In some embodiments, the fuel injected by the fuel injector is anyhydrocarbon compound that can be oxidized or any hydrocarbon compoundthat can be reduced. In some embodiments, the hydrocarbon fuel is agaseous, liquid, oxygenated, nitrogen containing, or sulfur containinghydrocarbon, or a mixture thereof. In other embodiments, the hydrocarbonfuel is gasoline or diesel fuel, or a mixture thereof.

In some embodiments, the catalytic zone includes at least one monolithicstructure. Often, the oxidation and reforming catalysts are applied tosurfaces of the monolithic structure as a washcoat, either separately orcombined. In some embodiments, the oxidation and reforming catalysts areapplied to the same area of the monolithic structure. In otherembodiments, the oxidation and reforming catalysts are applied todifferent areas of the structure. In one embodiment, the oxidationcatalyst is applied to an area that is upstream of the reformingcatalyst. In some embodiments, the monolithic structure includes aplurality of channels from the inlet face to the outlet face. In oneembodiment the monolithic structure includes metal. In anotherembodiment, the monolithic structure includes a ceramic material.

In some embodiments, the device is configured such that when rich andlean zones of an oxygen containing gas stream flow through the catalyticzone, the temperature of the catalytic zone is maintained at about 450to about 1000° C.

In one embodiment, the gas stream is heated prior to entry into theinlet of the catalytic zone by a heater or a heat exchanger upstreamfrom the catalytic zone, wherein the heater or heat exchanger is in gasflow communication with the catalytic zone.

In a still further embodiment, the device includes a pre-oxidationcatalyst downstream from the fuel injector and upstream from thecatalytic zone of the fuel processor. The pre-oxidation catalystincludes an oxidation catalyst and the fuel injector is configured tointroduce a fuel into at least a portion of a gas stream flowing throughthe pre-oxidation catalyst, such that when the gas stream flows throughthe pre-oxidation catalyst, at least a portion of the fuel introduced bythe fuel injector is oxidized, thereby heating the gas stream. In oneembodiment of a device with a pre-oxidation catalyst, the deviceincludes a mixer downstream from the pre-oxidation catalyst and upstreamfrom the catalytic zone, and the device is configured such that aportion of the fuel introduced by the fuel injector and flowing throughthe pre-oxidation catalyst is vaporized, and wherein the mixer isconfigured to mix the vaporized fuel in the gas stream in apredominantly radial fashion without substantial axial mixing. In oneembodiment of a device with a pre-oxidation catalyst, the pre-oxidationcatalyst is coated on at least a portion of the inner walls of afraction of the channels of a monolithic catalyst structure. In oneembodiment, the fraction of channels containing the coated catalyst isabout 20 to about 80%.

In another aspect, the invention provides a device for reducing NO_(X)content in oxygen-containing emissions of a lean burn engine. The deviceincludes a fuel injector and a first catalytic zone that includes anoxidation catalyst and a reforming catalyst. The fuel injector isconfigured to inject fuel into at least a portion of the oxygencontaining exhaust stream from a lean burn engine upstream from thecatalytic zone to provide rich and lean zones in the exhaust stream whenthe exhaust stream flows through the first catalytic zone. The devicefurther includes a second catalytic zone downstream from the firstcatalytic zone that includes a catalyst capable of reducing NO_(X) to N₂in the presence of a reducing gas. In one embodiment, the device isconfigured such that as a rich zone in an oxygen containing gas streamflows through the first catalytic zone, a portion of the fuel in therich zone is oxidized on the oxidation catalyst and at least a portionof the remaining fuel in the rich zone is reformed on the reformingcatalyst, thereby producing a reducing gas stream, and wherein thedevice is configured such that at least a portion of the exhaust streamand at least a portion of the reducing gas stream produced in the firstcatalytic zone flow through the second catalytic zone, such that whenthe exhaust stream and reducing gas streams flow through the secondcatalytic zone, NO_(X) is reduced to N₂ on the catalyst containedtherein.

In some embodiments, lean zones do not contain added fuel. In otherembodiments, lean zones contain some added fuel at an equivalence ratioless than 1. In embodiments in which lean zones contain some added fuel,preferably essentially all of the added fuel in a lean zone is oxidizedon the oxidation catalyst in the first catalytic zone or on apre-oxidation catalyst as described above.

The device may further include a reservoir that includes a hydrocarbonfuel and that is in fluid communication with the fuel injector, whereinthe reducing gas stream produced in the first catalytic zone includes H₂and CO.

In one embodiment, the fuel injector is adapted to introduce a fuel toan oxygen containing gas stream discontinuously to for alternating richand lean zones in the gas stream upstream of the first catalytic zone.In another embodiment, the fuel injector is adapted to introduce thefuel to a portion of an oxygen containing gas stream essentiallycontinuously to form a rich zone, and the device is configured such thatthe portion of the first catalytic zone through which the rich zoneflows varies over time.

In some embodiments, the lean burn engine is a diesel engine. In oneembodiment, the fuel is a hydrocarbon fuel, such as diesel fuel. Inembodiments in which the added fuel is a hydrocarbon fuel, the productsof the catalytic reforming reaction are H₂ and CO, which are used asreducing agents to reduce NO_(X) to N₂ in the second catalytic zone. Insome embodiments, the second catalytic zone includes a lean NO_(X)catalyst. In some embodiments, the device is configured such that aportion of the exhaust stream is diverted as a slipstream upstream fromthe first catalytic zone and the fuel injector is configured to injectfuel into the slipstream upstream from the first catalytic zone. Often,such a device with a slipstream is configured to divert about 5 to about25% of the exhaust stream by volume into the slipstream.

In one embodiment, the device further comprises a pre-oxidation catalystupstream of the first catalytic zone, the fuel is injected upstream ofthe pre-oxidation catalyst, and a portion of the added fuel is oxidizedon the pre-oxidation catalyst, thereby heating the gas stream, asdescribed above.

In some embodiments, the device includes a controller which controls theinjection of fuel as a function selected from exhaust NO_(X)concentration, exhaust O₂ concentration, engine rpm, engine torque,engine turbocharger boost, engine intake air flow rate, exhaust intakeflow rate, exhaust flow rate, and exhaust temperature, or a combinationthereof. In one embodiment, the injection of fuel is controlled as afunction of exhaust NO_(X) concentration, which is determined by atleast one NO_(X) sensor in the exhaust stream. In some embodiments, thedevice is downstream from a lean burn engine that includes an enginecontrol unit, and the controller is incorporated into the engine controlunit.

In another aspect, the invention includes a process for producing areducing gas which includes introducing a fuel into at least a portionof an oxygen containing gas stream to create rich and lean zones in thegas stream, wherein a portion of the fuel in a rich zone is oxidized andwherein at least a portion of the remaining fuel in the rich zone isreformed, thereby producing a reducing gas. In one embodiment, the richand lean zones flow through a catalytic zone that includes an oxidationcatalyst and a reforming catalyst. A portion of the fuel in a rich zoneof the gas stream is oxidized on the oxidation catalyst and at least aportion of the remaining fuel in the rich zone is reformed on thereforming catalyst to produce a reducing gas stream. When a hydrocarbonfuel is used, the reducing gas stream includes H₂ and CO.

In another aspect, the invention includes a process for reducing NO_(X)content in oxygen containing emissions of a lean burn engine, e.g., adiesel engine, which includes introducing a fuel, e.g., diesel fuel,into at least a portion of the oxygen containing exhaust stream from thelean burn engine to create rich and lean zones in the exhaust stream,wherein a portion of the fuel in a rich zone is oxidized and wherein atleast a portion of the remaining fuel in the rich zone is reformed,thereby producing a reducing gas. At least a portion of the reducing gasis used to reduce NO_(X) to N₂ on a catalyst. In one embodiment, therich and lean zones in the exhaust stream flow through a first catalyticzone downstream from the fuel injector and having an oxidation catalystand a reforming catalyst, wherein a portion of the fuel in a rich zoneof the exhaust stream is oxidized on the oxidation catalyst and at leasta portion of the remaining fuel in the rich zone is reformed on thereforming catalyst, thereby producing a reducing gas. At least a portionof the reducing gas is introduced into at least a portion of the exhauststream flowing through a second catalytic zone downstream from the firstcatalytic zone, wherein the second catalytic zone includes a catalystcapable of reducing NO_(X) to N₂ in the presence of the reducing gas,and wherein NO_(X) is reduced to N₂ in the second catalytic zone. In oneembodiment, the fuel is a hydrocarbon fuel, such as a diesel fuel, andthe reducing gas includes H₂ and Co.

In one embodiment, the portion of the first catalytic zone flowingthrough the first catalytic zone is diverted as a slipstream upstreamfrom the first catalytic zone, and the fuel is injected into theslipstream.

In one embodiment, the fuel is injected upstream of a pre-oxidationcatalyst, which is upstream from the first catalytic zone, and a portionof the injected fuel is oxidized on the pre-oxidation catalyst, therebyheating the gas stream, as described above.

In another aspect, a device as described above is adapted for use in avehicle having a lean burn engine. In one embodiment, the lean burnengine is a diesel engine. A vehicle may include a device of theinvention in contact with at least a portion of the exhaust stream froma lean burn engine on a vehicle, such as a diesel engine. In oneembodiment, the fuel injected by the fuel injector is diesel fuel onboard a vehicle with a diesel engine.

In one aspect, the invention provides a vehicle that contains a devicefor H₂ production and/or NO_(X) emission as described above. Forexample, a device as described above may be situated in the exhauststream of a vehicle, such as an automobile, truck, commercial vehicle,airplane, etc., with at least a portion of the exhaust stream flowingthrough the device, which is situated downstream from the engine of thevehicle. Methods for reducing NO_(X) emission as described above may beprovided by placing a device of the invention as described above in theexhaust stream of a vehicle, downstream from the engine of the vehicle.The engine of the vehicle may be a SI or CI lean burn engine. In oneembodiment, the engine is a lean burn diesel engine.

In another aspect, a device or process of the invention may be used inconjunction with an engine used for stationary power generation or todrive a mechanical device.

In a still further aspect, the invention provides a catalyst compositionfor producing H₂ and CO which includes an oxidation catalyst and areforming catalyst. In one embodiment, the oxidation and reformingcatalysts include the same catalytically active component(s). In anotherembodiment, the oxidation and reforming catalysts include differentcatalytically active component(s).

In another aspect, the invention provides a monolithic structure onwhich an oxidation and reforming catalyst composition or compositionshave been coated, either in different or the same areas of thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts the steady state temperature of a fuelprocessing reactor at different equivalence ratios (Φ).

FIG. 2 depicts one embodiment of a fuel processor in accordance with theinvention, containing an exhaust inlet 1 and outlet 10, a fuel injector3, a catalytic zone 6 for oxidation and reforming of injected fuel, andmixing zones 4 and 9. The mixing volumes 4 and 9 may be the same ordifferent sizes and may optionally include internal components to aid orimprove mixing.

FIG. 3 schematically depicts various parameters relating to a fuelprocessor of the invention wherein fuel is added in a pulsed manner.FIG. 3 a depicts fuel flow over time when fuel is added to an exhauststream in a pulsed manner. FIG. 3 b depicts concentration of O₂ overtime as added fuel is catalytically combusted. FIG. 3 c depicts thetemperatures of the inlet and outlet of the catalytic zone of the fuelprocessor over time. FIG. 3 d depicts concentration of H₂ and CO overtime as added fuel is catalytically reformed. FIG. 3 e depictsconcentrations of O₂, H₂, and CO after mixing.

FIG. 4 depicts one embodiment of a NO_(X) emission control device of theinvention. A portion of the exhaust stream 40 is diverted as aslipstream 41. Fuel injected through injector 42 reacts with the exhaustgases in a first catalytic zone 44 to produce H₂ and CO, which are thenmixed 46 and 53 into the flowthrough exhaust. H₂ and CO react withNO_(X) in a second catalytic zone 48 to produce N₂.

FIG. 5 depicts one embodiment of the fuel processor of the invention. InFIG. 5A, the catalytic zone 64 comprises a plurality of channels, andfuel is added continuously, mixed with the flowing exhaust, and thenadded to a portion of the rotating catalyst structure. In FIG. 5B, fuelis injected into through fuel injector 71 and into a flow guide 70. InFIG. 5C, a portion of the exhaust stream 83 bypasses the rotatingcatalyst 85.

FIG. 6 shows the results of an experiment in which pulses of fuel wereused to catalytically produce H₂ in the presence of 7% O₂—FIG. 6A showsO₂ consumption and H₂ production at different frequencies of fuelinjection. FIG. 6B shows the inlet and outlet temperatures of thecatalyst structure over time.

FIG. 7 shows the effect of mixing on H₂ concentration over time. FIG. 7Ashows the concentration of H₂ produced as a result of added pulses offuel in the presence of 7% O₂, prior to mixing. FIG. 7B shows H₂concentration after mixing.

FIG. 8 depicts calculated values of fuel penalty versus stoichiometryfactor for several levels of exhaust gas flow diverted to a fuelprocessor of the invention.

FIG. 9 depicts temperature versus time as fuel is pulsed through thecatalyst of a fuel processor of the invention, for a catalyst mass togas flow ratio of 1250 liters per minute, catalyst mass 1000 g, pulsefrequency 0.4 Hz, equivalence ratio of approximately 3, and 10% O₂.

FIG. 10 depicts temperature oscillation amplitude vs. frequency ofoperation for several different catalyst mass to gas flow ratios.

FIG. 11 depicts an embodiment of a NO_(X) emission control deviceincluding a fuel injector 91, a pre-oxidation catalyst 92, a mixer 93,and oxidizing and reforming catalysts 94.

FIG. 12 shows the results of an experiment to compare the efficiency ofNO_(X) conversion using pulsed versus continuous addition of reducingagents H₂ and CO to a flowing gas stream through a lean NO_(X) catalyst.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides methods and devices for improving emissioncontrol in lean-burn engines. In particular the invention provides forproduction of H₂ and CO at high efficiency from a hydrocarbon fuelsource, and use of these reducing agents for catalytic reduction ofNO_(X) to N₂ in the presence of excess O₂. This process is applicable toany engine that produces an exhaust stream containing excess O₂. In oneembodiment of the invention, the fuel on board a vehicle is used toproduce H₂ and CO, which then serve as reducing agents to convert theNO_(X) in the vehicle's exhaust stream to N₂. The invention alsoprovides methods and devices for production of a reducing gas streamfrom a fuel in an oxygen containing environment.

Production of H₂ from Hydrocarbon Fuel

The invention provides a fuel processing device and methods forproducing a reducing gas containing H₂ and/or H₂ and CO in an O₂containing environment. The reducing mixture produced from a fuelprocessing device of the invention may be used in a process for controlof NO_(X) emission as described below, or for other applications such asstabilization of the combustion flame in a burner or selective removalof other pollutants.

Devices of the invention employ a fuel processor to produce a reducinggas mixture in an O₂ containing gas stream. In some embodiments, the O₂containing gas stream is the exhaust stream from a diesel engine, whichgenerally contains O₂, CO₂, H₂O, and NO_(X).

The fuel processor includes a catalytic zone that contains oxidation andreforming catalysts. The catalysts are in contact with at least aportion of the gas stream.

The fuel processor also includes a fuel injector for addition of a fuelto the oxygen containing gas stream. The fuel injector introduces a fuelto the gas stream in a manner such that rich and lean zones are formedin the gas stream flowing through the catalytic zone. In someembodiments, the added fuel is a hydrocarbon fuel.

“Equivalence ratio” as used herein refers to the ratio between actualamount of fuel and the theoretical stoichiometric amount of fuel whichwould be required to fully react with all of the O₂ present in a gasmixture. As used herein, “lean” refers to a fuel air equivalence ratioless than 1.0 and “rich” refers to a fuel air equivalence ratio greaterthan 1.0. A rich zone is produced in the flowing gas stream when fuel isadded such that the equivalence ratio in the portion of the gas streamto which the fuel has been added has an equivalence ratio greaterthan 1. A lean zone is produced either when no fuel is added or whenfuel is added in an amount such that the equivalence ratio in theportion of the gas stream to which the fuel has been added is less than1.

In some embodiments, production of alternating lean and rich conditionsin an O₂ containing gas stream is accomplished by adding a fuel, such asa hydrocarbon fuel, in a pulsed, discontinuous manner to formalternating rich and lean zones in the gas stream. In other embodiments,fuel is added essentially continuously to form a rich mixture in aportion of the flowing gas stream as the portion of the catalytic zonethrough which the rich mixture flows is continuously or periodicallyvaried over time.

Under rich conditions, a portion of the added fuel is combusted withoxygen in the gas stream on the oxidation catalyst to release heat, andat least a portion of the remaining added fuel reacts with H₂O producedin the combustion reaction and/or present in the gas stream on thereforming catalyst to produce H₂, an endothermic reaction. Thecombustion reaction provides heat to raise the temperature of thereforming catalyst to an appropriate level to efficiently reform theadded fuel. When a hydrocarbon fuel is used, the products of thereforming reaction are H₂ and CO, which may both serve as reducingagents in a downstream process such as reduction of NO_(X) on a leanNO_(X) catalyst. When ammonia is used as the added fuel, the products ofthe reforming reaction are hydrogen and nitrogen or nitrogen oxides.

Under lean conditions, in embodiments in which lean zones contain noadded fuel, no combustion or reforming reactions due to added fuel takeplace in the catalytic zone of the fuel processor.

In other embodiments, some fuel is added at an equivalence ratio lessthan 1 to create lean zones in the oxygen containing gas stream, togenerate additional heat. A desirable amount of fuel in the lean zonesin such embodiments is determined by the required temperature of thefuel processor catalysts. Additional heat may be required in surplus ofthe heat generated by combustion of oxygen in the rich zones in order tomaintain the catalytic zone at the required operating temperature, forexample if the rich zones are very small in magnitude or short induration, or if the oxygen level in the gas stream is low. In suchembodiments in which some fuel is added to form lean zones in theoxygen-containing gas stream at an equivalence ratio less than 1,essentially all of the fuel in the lean zone is combusted with theoxygen in the gas stream on the oxidation catalyst to produce additionalheat. Preferably, the amount of additional heat produced in such anembodiment is generally an amount sufficient to maintain the temperatureof the catalytic zone within the desired operating temperature range.

The products of the reforming reaction in rich zones of the gas stream,for example H₂ and CO when a hydrocarbon fuel is used, exit thecatalytic zone with the other components of the gas stream, and mayoptionally be used in a downstream process to reduce NO_(X) to N₂ on alean NO_(X) catalyst for emission control.

In some embodiments, the reducing gas produced by the fuel processor maybe fractionated to produce a substantially pure H₂ stream. Separationtechniques such as, for example, distillation, pressure swingabsorption, or use of a selective membrane (e.g., a microporous membranethrough which H₂ diffuses) may be used to separate H₂ front othercomponents of the reducing gas mixture produced by the reformingcatalyst.

General Description of the Fuel Processor

As described above, the invention provides a fuel processor forproducing H₂ from added fuel in an O₂ containing gas stream. The exhaustfrom a lean burn engine, such as a diesel engine, typically contains8-15% O₂ and 6-10% H₂O. A fuel processor of the invention may be used toproduce reducing agents, e.g. H₂ and CO from added hydrocarbon fuel, insuch an exhaust stream, which may be used as reducing agents in adownstream process such as reduction of NO_(X) emission.

To avoid the disadvantages of continuous fuel addition discussed above,the fuel processing device of the present invention includes a fuelinjector for adding a fuel such that the gas stream flowing over thecatalyst alternates between rich and lean conditions. When used in themanner described herein, this results in a lower temperature for thefuel processor catalyst and a lower temperature for the gas mixtureexiting the fuel processor catalyst than if the fuel were addedcontinuously.

One illustrative embodiment of a fuel processor device of the inventionis depicted in FIG. 2. FIG. 2 shows a pipe or duct 1 that is connectedto a fuel injector 2 and a mixing system 4. A gas stream flows into thesystem through pipe 1, and is mixed with fuel injected through the fuelinjector 2 fed by fuel supply 3. Fuel is injected into the gas streamupstream of the inlet of the catalytic zone 6. In one embodiment,depicted in FIG. 3, a hydrocarbon fuel is injected into mixer 4 in adiscontinuous, pulsed manner to form rich zones when the fuel injectoris injecting fuel into the gas stream and lean zones when the fuelinjector is not injecting any fuel into the gas stream.

As fuel, for example hydrocarbon fuel, is added in this manner, asdepicted in FIG. 3 a, rich and lean zones are created in the gas stream,such that the equivalence ratio in a rich zone is greater than 1 and theequivalence ratio in a lean zone is less than 1. The resulting rich andlean gas mixtures flow through a catalytic zone 6, as shown in FIG. 2,which contains oxidation and reforming catalysts, where the hydrocarbonfuel in a rich zone of the gas stream is combusted with O₂ in the gasstream on the oxidation catalyst, and then converted to H₂ and CO on thereforming catalyst after essentially all of the O₂ has been consumed.

During the period when the fuel is injected to form a rich zone, thefuel flow is set at a flow rate such that the equivalence ratio in theportion of the gas stream into which fuel is injected is above 1,typically at least about 1.5, often at least about 2. Since theequivalence ratio in the rich zone is above 1, essentially all of the O₂is consumed, as shown in FIG. 3 b, and the O₂ level exiting thecatalytic zone is essentially zero, i.e., close to or equal to zero. Thecombustion of the large amount of oxygen in the gas stream raises thecatalyst and gas temperature to the level needed to cause the reformingof the remaining fuel as shown in FIG. 3 c. Since the fuel-exhaustmixture is “rich,” i.e., equivalence ratio greater than 1 when the fuelis being injected, and since the temperature is high due to thecombustion of a portion of the fuel, H₂ and CO are formed from theexcess fuel, as shown in FIG. 3 d. A fuel pulse is sufficiently longthat the resulting heat that is generated by full combustion of O₂ heatsthe oxidation and reforming catalysts 6 in FIG. 2 to the desiredtemperature for reforming, generally about 450 to about 1000° C.,sometimes about 500 to about 900° C., often about 550 to about 650° C.,most often about 600° C. Before the catalyst temperature can rise to thesteady state temperature, which would be too high for good durability ofthe catalyst, the fuel is shut off (region 12 in FIG. 3 a).

As low temperature gas in a lean zone of the gas stream flows throughthe catalytic zone, the catalyst temperature decreases. Subsequent fuelpulses can be added, as shown in FIG. 3 a, with the fuel flow during theperiod of fuel injection controlled to reach the desired equivalenceratio. As the fuel is pulsed in this manner, pulses of H₂ and CO aregenerated, as shown in FIG. 3 d, in the exhaust stream exiting theoutlet of the catalytic zone, while the catalyst temperature ismaintained at a relatively constant level, as shown in FIG. 3 c.

Exhaust stream 7 is optionally passed through a mixer, e.g., 8 in FIG.2, to generate a relatively steady state concentration of H₂ and CO, asrepresented in FIG. 3 e. Mixers may optionally be provided upstream ofthe catalytic zone, to provide a uniform fuel-exhaust mixture, and/ordownstream of the catalytic zone, to provide a uniform concentration ofreducing agent in the exhaust stream. Such mixers, represented byreference numerals 4 and 8 in FIG. 2, can be the same or differentsizes, can be configured in a variety of shapes and can include internalstructures or devices to promote mixing, such as veins, tabs, or otherphysical devices that do not induce a large axial recirculation zone.Appropriate internal mixers may be readily determined by one of skill inthe art. If pulsing or discontinuous reductant, e.g., H₂ and CO,concentration is desired, mixer 8 of FIG. 2 may be eliminated.

In one embodiment, a first pulse of fuel is longer than subsequentpulses, to rapidly heat the catalysts to the desired temperature, asdepicted in FIG. 3 a. Subsequent pulses can be of a selected flow rate,duration and frequency to maintain a relatively constant catalysttemperature. In another embodiment, the fuel pulsing is initially set ata desired steady state frequency with all pulses of essentially equalduration, with the result that the catalyst temperature will graduallyreach the steady state temperature and the H₂ and CO output willgradually approach the desired value. In the embodiment shown in FIG. 3,the fuel flow during the lean portion 12 of a cycle is essentially zero.In other embodiments, some fuel is injected during the lean part of thecycle at an equivalence ratio less than 1, so that additional heat canbe generated during this portion of the cycle as the added fuel isconsumed in a combustion reaction on the oxidation catalyst.

The fuel processor of the invention is not limited by the illustrativeembodiments described above. Alternate designs for the fuel processormay be used in accordance with the present invention so long as they arecapable of adding fuel to an oxygen-containing gas stream to form richand lean zones in the gas stream for efficient reforming of the fuel asdiscussed above.

In some embodiments, rather than adding the fuel in a pulsed manner,fuel is added essentially continuously with rich and lean zones formedin the gas stream by radially rotating the catalyst structure relativeto the direction of flow of the gas stream, or by continuously orperiodically changing the position or spray angle of the fuel injectorsuch that only a portion of the catalytic zone is in contact with a richmixture at any given time and the portion in contact with the richmixture varies over time.

In one embodiment of the fuel processor, depicted in FIG. 5A, fuel isadded continuously to a portion of the gas stream entering a rotatingcatalyst structure having a plurality of longitudinal channels,configured such that the gas stream flows through the channels from theinlet to the outlet of the catalyst structure. The walls of the channelsare coated with oxidation and reforming catalysts. The radial rotationof the catalyst structure within the gas stream, while fuel iscontinuously added to a portion of the gas stream entering the catalyststructure, effectively produces the same effect as adding fuel in pulsesto produce alternating rich and lean zones flowing through the entirecatalytic zone. By using an appropriate geometry, periodic lean and richconditions are created in continuously or periodically varying portionsof the catalyst structure. The exhaust gas enters fuel processor 60through exhaust duct 61, passes through main chamber 62 and out exhaustduct 63. Within the main chamber 62 is a monolithic catalyst substrate64 with longitudinal channels, for example in a honeycomb configurationas shown in FIG. 5A, that is caused to rotate by a shaft 65 driven bydrive unit 66. Fuel is injected through fuel injector 67, creating aspray pattern 68 and thus a rich zone that encounters the monolithicstructure in the region shown by outline 69. The continuous fuelinjection rate is set so that the equivalence ratio in region 69 isabove 1, typically in the range of between about 2 to about 5. Region 69of the catalytic zone is varied over time as the catalyst structurerotates.

In another embodiment, the fuel is continuously injected into a flowguide 70 as depicted in FIG. 5B. Fuel injector 71 injects the fuel intothe inlet of the flow guide and the flow guide allows mixing of the fueland exhaust gas to produce a relatively uniform fuel concentration asthis fuel exhaust mixture enters catalyst 72.

The fuel processor embodiments depicted in FIG. 5 a and FIG. 5 b do notrequire a tight gas seal between the catalytic zone (64, 72) and themain chamber (62, 73). This greatly simplifies the design of therotating catalyst (64, 72). Rotating the catalyst structure addscomplexity, additional components, and added cost to the system but thisapproach has the advantage of producing an essentially continuous streamof exhaust containing reducing agents, for example H₂ and CO when ahydrocarbon fuel is added to the gas stream, that is more readily mixedwith the remaining exhaust or gas flow.

In an alternative implementation, rather than rotating the catalyststructure, the spray angle of fuel injector 67 is changed. By changingthe spray angle, the region of rich fuel/air ratio may be moved aroundon the catalyst structure, effectively duplicating the operation of arotating catalyst as in FIG. 5A. The injector spray angle may be changedmechanically by physically moving the injector, by using an additionalair flow that interacts with the fuel spray from the injector to changethe effective spray angle, or by using electrically driven componentswithin the injector to change the spray angle.

The fuel processor 60 shown in FIG. 5A or FIG. 5B can be configured inan emission control system where a portion of the exhaust stream isdiverted as a slipstream to the fuel processor and then reintroducedinto the main exhaust stream after generating products of the reformingreaction, such as H₂ and CO. An alternative approach is to allow most ofthe exhaust stream to bypass the catalyst structure as shown in FIG. 5C.The entire exhaust flow 80 enters the fuel processor through duct 81. Alarge portion of the exhaust bypasses the fuel processor catalyst asshown by flow path 83 and exits fuel processor unit 82 through duct 84.A portion of the exhaust flow is diverted to the fuel processor catalyst85, dependent upon the flow resistance for each path. The fuel injectionand fuel processor rotating catalyst structure or varying fuel sprayangle operate essentially as described for the fuel processor systemsdepicted in FIG. 5A and FIG. 5B. In some embodiments, the fuel processorunit 82 includes baffles and partitions inside the main chamber 86 todirect the desired amount of flow through the fuel processing catalyst.Appropriate design and configuration of such partitions and baffles maybe readily determined by those skilled in the art.

In embodiments such as those represented in FIGS. 5A, 5B, and 5C, thecatalyst structure is preferably be of sufficient thermal mass tomaintain the catalysts at the desired operating temperature, generallyabout 450 to about 1000° C., sometimes about 500 to about 900° C., oftenabout 550 to about 800° C., often about 600° C.

A number of hydrocarbon fuels are suitable for addition to theoxygen-containing gas stream in any of the fuel processor embodiments ofthe invention, including diesel fuel, gasoline, methane, kerosene, otherhydrocarbons, alcohols, or any hydrocarbon containing fuel. Gaseous,liquid, oxygenated, nitrogen containing, and sulfur containinghydrocarbons may also be used. In addition, a non-carbon containing fuelsuch as ammonia, hydrogen sulfide, or other combustible material, may beused if combustion at an equivalence ratio greater than 1 produces ahydrogen containing stream in a device of the invention. The fuel usedmust be capable of releasing an appropriate amount of heat uponcombustion to raise the temperature of the reforming catalyst to a levelsuitable for efficient production of reducing agents, e.g., H₂ or H₂ andCO. In a particularly advantageous embodiment, using this approach, ahydrocarbon fuel on board a vehicle with a lean burn engine can beprocessed with the engine exhaust to produce an exhaust streamcontaining H₂ and CO.

NO_(X) Emission Control

The invention provides an emission control device and method forreducing the NO_(X) content in oxygen-containing emissions from acombustion process, particularly combustion of a hydrocarbon fuel thatoccurs in a lean burn internal combustion engine. Devices of theinvention are particularly useful for reducing NO_(X) emissions in theexhaust of a vehicle diesel engine. As used herein, “NO_(X)” refers tonitrogen oxides produced in a combustion process, particularly nitrogenoxides present in the exhaust stream of an internal combustion engine,such as NO and NO₂. A “lean burn” or “lean burning” engine refers to anengine that combusts hydrocarbon fuel at an air to fuel ratio in whichthere is more air than the stoichiometric amount of air needed tooxidize the fuel. This requires an air to fuel mass ratio above 15 andtypically above 25 for a diesel engine. Emissions from a lean burndiesel engine typically contain about 8-15% O₂ and 400-700 ppm NO_(X) inthe exhaust gas.

The emission control device also includes a second catalytic zone incontact with the exhaust stream and downstream from the catalytic zoneof a fuel processor as described above. The second catalytic zoneincludes a catalyst composition that includes a catalyst, such as a“lean NO_(X) catalyst,” which is capable of selective catalyticreduction of NO_(X) to N₂ in an O₂ containing environment, particularlythe exhaust of a lean burn engine, using the H₂ and CO produced by thefuel processor. When the reducing agent(s) produced by the fuelprocessor, e.g. H₂ and CO, reach the second catalytic zone, NO_(X) isreduced to N₂ on the catalyst therein, thereby providing a reduction inNO_(X) emission.

The invention provides methods for reducing NO_(X) produced by a leanburn combustion process, such as combustion in a diesel engine, using anemission control device of the invention. Methods of the inventioninclude injecting a fuel into at least a portion of an O₂ and NO_(X)containing exhaust stream to create rich and lean zones in the exhauststream. The rich and lean zones flow through the catalytic zone of thefuel processor as described above. The catalytic zone of the fuelprocessor contains both oxidation and reforming catalysts. In richzones, a portion of the fuel is oxidized and at least a portion of theremaining fuel is reformed on the reforming catalyst to produce reducingagent(s), for example H₂ and CO when a hydrocarbon fuel is used. Thereducing agents, e.g., H₂ and CO, are introduced into the exhaust streamupstream from a second catalytic zone and react with NO_(X) on acatalyst in the second catalytic zone to produce N₂, thereby reducingNO_(X) emission.

In an advantageous embodiment, the fuel on board a vehicle with a leanburn engine, for example diesel fuel on board a diesel engine poweredvehicle, is processed in a fuel processor of the invention to produce H₂and CO, which reduce NO_(X) emission levels in conjunction with a leanNO_(X) catalyst in the second catalytic zone of an emission controldevice as described herein.

In one embodiment, depicted schematically in FIG. 4, a portion of anoxygen-containing engine exhaust stream 40 is diverted as a “slipstream”41. Fuel is then injected into the gas stream in the slipstream througha fuel injector 42. The fuel is added to the slipstream exhaust inregion 43 and the mixture then flows through the catalytic zone 44 toproduce products of the reforming reaction, e.g., H₂ and CO, which isthen mixed with the flowthrough engine exhaust stream in mixers 46 and53. The reforming products, e.g., H₂ and CO, then react with the NOx inthe exhaust stream on a lean NO_(X) catalyst 48 to reduce the NO_(X)emission level in the exiting exhaust stream 49.

In an embodiment in which a portion of the exhaust stream is diverted ina slipstream, the amount of exhaust that is diverted to the slipstreamand through the fuel processor catalytic zone can be varied over a widerange. Typically, about 1 to about 50% of the total exhaust is divertedinto the slipstream. Since the gas flow through the fuel processor isheated to the reforming temperature by oxidizing fuel, this represents aheat loss and decreased efficiency. For this reason, it is preferable tolimit the amount of the exhaust stream that is diverted to the fuelprocessor. The reforming products, e.g., H₂ and CO, generated by thefuel processor are then added back to the main exhaust flow at 52resulting in a large dilution. This large dilution means that for agiven target concentration in the mixed stream 47, the concentration ofreductant produced at 45 must be proportionately higher. For example,for 1000 ppm of H₂ and CO at the inlet to the second catalytic zone 47and with a split of 5% to the fuel processor, the average concentrationof H₂ and CO at 45 must be 2%. These two requirements drive the splitratio in different directions.

FIG. 8 shows some calculated values of fuel penalty versus stoichiometryfactor for several levels of exhaust gas flow diverted to the fuelprocessor. These calculations were done for a typical diesel engine witha NOx emission level in the range of 5 g/b-hp-hr (grams per brakehorsepower hour) and operating at high load. The stoichiometry factor isthe ratio of H₂ and CO that is required to reduce each molecule of NOx.At a stoichiometry factor of 1, one H₂ molecule or one CO molecule willreduce one NOx molecule. As shown, the lowest fuel penalty is obtainedwith the lowest flow of exhaust diverted to the fuel processor and thelowest stoichiometry factor. The theoretical stoichiometry factor wouldbe 1 for NO and 2 for NO₂ and for an efficient lean NOx catalyst thestoichiometry factor would be expected to be in the range of 1 to 3.Preferably, the fraction of exhaust flow diverted to the fuel processorranges from a volume percentage of about 1% to at least about 50%, morepreferably from about 3 to about 30%, even more preferably from about 5to about 25%, most preferably about 8 to about 15%.

Since the fuel processor catalyst structure 44 in FIG. 4 may provideresistance to flow of the diverted exhaust stream, some embodimentsinclude a means of regulating the “split ratio,” or ratio of amount ofexhaust diverted into the slipstream to amount of flowthrough exhaust.Examples of means suitable for regulating the split ratio includevariable means, such as a valve at location 51 or 52. In someembodiments, the valve includes a door that is fixed or variable,although other means capable of directing a certain amount of exhaustflow to the fuel processor 50 may be used in accordance with theinvention. A fixed regulation means may also be used. One example of afixed means is a flow restrictor, such as an orifice, that is located inthe main exhaust duct between locations 51 and 52. Such a restrictionwould direct a given fraction of the exhaust flow to the fuel processor50 based on the relative restriction caused by the flow restrictor inthe main duct compared to the restriction caused by the components offuel processor 50. A restrictor placed in the duct between locations 51and 52 can be either a fixed restrictor with a fixed opening or it canbe a variable restrictor such as a valve or a door.

Fuel Injection

A number of fuel injectors suitable for use in the invention are wellknown in the art. In some embodiments of the invention, pressurized fuelis supplied to the injector and the injector then opens and closes aflow control valve to turn the fuel flow on and off. Such injectors havebeen extensively developed as automotive fuel injectors and aredescribed, for example, in U.S. Pat. Nos. 6,454,192, 5,979,866,6,168,098, and 5,950,932. Such injectors utilize a low pressure supplyof fuel, in the range of 30 to 600 psig and can turn fuel flow on andoff very rapidly, at a speed that is typically in the range of 0.2 to 1millisecond, using an electrical signal to move a solenoid or valvewithin the injector. Such fuel injectors control the fuel flow rate byopening and closing the injector very rapidly, with the fraction of timeopen set to control the fuel flow. For use in devices of the invention,the frequency of this opening and closing can be very fast, so that thefuel flow is essentially continuous. For example, using a frequency of50 to 100 Hz and controlling the fraction of time open, the fuel flowrate can be controlled to produce the desired equivalence ratio duringthe rich pulses shown in FIG. 3 a. The fuel injector is then fullyclosed during the lean periods. Thus, such fuel injectors may beoperated with two frequency components, a high frequency component thatwould be used to give the required fuel flow rate during the rich andlean periods shown in FIGS. 3 a and 3 d. The injector would beessentially off for the lean periods shown in FIG. 3 a and FIG. 3 d. Ifsome fuel flow is desired during the lean periods to maintain fuelprocessor catalyst temperature, then high frequency operation of theinjector with a very low fraction of open time would result in a verylow fuel flow.

Other types of injectors may also be used, such as air assist injectors.Air assist injectors utilize pressurized air, which flows through theinjector with the fuel to obtain a desired fuel droplet size, spraypattern, or spray direction, or to utilize fuel at a lower supplypressure. Multiple injectors may also be used. As another alternative,the injection means may include a simple nozzle to disperse and directthe fuel spray, this nozzle being connected to a fuel line whichsupplies the fuel in pulses from a pulse pump or other means. Theinjected fuel must be transferred to the fuel processor catalyst in ashort time and it would be disadvantageous for it to remain in liquidform on pipe or component walls. One embodiment which provides asolution to this problem includes direct spraying of the fuel onto thecatalyst surface, limiting the interaction of the fuel with potentiallycold metal surfaces of the fuel processor or the exhaust system. Anotherembodiment includes spraying of the fuel on a very hot surface to flashvaporize it. A further embodiment includes pre-vaporization of the fuelin a separate hot chamber, followed by release of the vaporized fuelthrough an injector.

Mixing of Fuel and Exhaust

To achieve the correct catalyst temperature for the generation ofreforming products, e.g., H₂ and CO, the fuel to oxygen ratio must bewithin an appropriate range to provide the desired level of heat outputto maintain the catalyst temperature and to produce the required levelof H₂ and CO. This requires mixing of the injected fuel and the exhaustflow upstream of the fuel processor catalyst to achieve an appropriatelevel of uniformity of the fuel-exhaust mixture prior to entering thecatalytic zone. Using variation of equivalence ratio to define the levelof uniformity required, an equivalence ratio uniformity of +/−40% isdesired, +/−30% is preferred, and a uniformity of +/−20% is mostpreferred. One method is to use a very uniform injector providing auniform fuel spray pattern to the inlet face of the catalyst structure.This uniform fuel spray pattern could then be combined with a uniformexhaust gas flow through the fuel processor catalyst, resulting in arelatively uniform fuel concentration at the fuel processor catalyticzone inlet. In some embodiments, the fuel and gas flow mixture enteringthe fuel processor catalytic zone include partially vaporized andpartially liquid fuel. When the fuel is not fully vaporized, contact ofthe fuel gas mixture with metal surfaces can result in collection of thefuel as a liquid film on these surfaces, which can reduce the effectivefuel concentration uniformity. In addition, slow evaporation of the fuelcan alter the lean and rich zones and reduce the performance of thesystem to produce reductant, e.g., H₂ and CO. Preferably, the fuelinjector and exhaust flow are configured to produce a substantiallyuniform fuel concentration at the inlet to the catalytic zone of thefuel processor. In some embodiments, at least some portion of the fuelentering the catalytic zone is not vaporized and enters in the form ofdroplets.

A second method to achieve the desired fuel concentration at the fuelprocessor catalytic zone inlet is to first partially react the fuel withoxygen on a pre-combustion catalyst thus raising the fuel-exhaustmixture temperature and partially or completely vaporizing the fuel.This vaporized fuel and exhaust mixture is then mixed using standard gasmixing techniques to form the desired equivalence ratio uniformity forthe fuel processor catalyst. Such a system is shown schematically inFIG. 11, including a fuel processor system 90 with a fuel injector 91injecting fuel onto pre-oxidation catalyst 92. The pre-oxidationcatalyst combusts a portion of the fuel with oxygen in the gas streamand raises the temperature such that some portion of the fuel isvaporized. This vaporized fuel is then mixed with the exhaust flow bymixer 93 to form a more uniform fuel oxygen mixture for the fuelprocessor oxidation/reforming catalyst 94 which then produces reductant,e.g., H₂ and CO, during the rich cycles. Mixer 93 is designed to mix thefuel and exhaust gas radially and to limit mixing of the fuel and airaxially thus maintaining the high equivalence ratio during the richpulse and thus maximizing reforming of fuel to H₂ and CO. Predominantlyradial mixing without substantial axial mixing is generally desirable tomaintain the magnitude of a rich pulse.

In one embodiment, the pre-oxidation catalyst is a washcoated monolithichoneycomb substrate with open channels to allow a low pressure drop. Thepre-oxidation catalyst substrate can be a ceramic or metal honeycombstructure with the channel walls coated with an oxidation catalyst. Thecatalyst substrate can be of any length and contain any channel size,but a short length substrate or a large channel size may be desirable insome embodiments since it is desirable to react only a portion of thefuel to raise the mixture temperature sufficiently to vaporize a portionor all of the fuel. The catalyst substrate structure can alternativelyinclude a metal substrate that is formed from corrugated metal stripsthat are coated with oxidation catalyst on only one side and then formedinto a spiral structure as described in U.S. Pat. Nos. 5,250,489 and5,512,250. Such structures with a catalyst coating on only one side ofthe wall of adjacent channels can limit the temperature rise of thecatalyst substrate. This would be desirable if the fuel-exhaust mixtureis not uniform. It is desirable that the amount of fuel vaporized insuch a pre-combustor catalyst system be at least about 50%, preferablyabout 70% and most preferably about 80% vaporized. In some embodiments,it is desirable to vaporize essentially all of the fuel to prevent itfrom collecting on the walls of the emission control system.

Operation at Low Exhaust Temperature

At low exhaust temperatures the injected fuel may not be sufficientlyvaporized. To overcome this limitation, an electric heater can be placedupstream of the fuel processing catalyst to heat the portion of exhaustflow into the fuel processing catalyst. For example in the embodimentdepicted in FIG. 4, an electric heater could be placed in duct 41 toheat the exhaust gas flowing into the fuel processor 50, through fuelair mixing space 43 and into fuel processor catalyst 44. This electricheater can be of any suitable type known in the art. For example, itcould consist of electrical resistance wire suspended in the exhaustflow, electrically heated metal strips, electrically heated metal wallsof the flow path of the exhaust stream or any method of heating theexhaust gas to the desired temperature.

An alternative method is to electrically heat a portion of the fuelprocessor catalyst by employing a catalyst substrate made of metal andpassing a current through the metal substrate. The electrical powercould be limited by using a large channel size so that only a portion ofthe exhaust gas flow is heated directly by the heated metal substrate. Afurther alternative is to use heat exchange between the hot gas exitingthe fuel processor catalytic zone (45 in FIG. 4) and the gas enteringthe fuel processor system (41 in FIG. 4). This heat exchange can beperformed by a number of methods that are well known in the art,including, for example, tube and shell heat exchangers, fin and tubeheat exchangers, or pipe devices.

Oxidation and Reforming Catalysts

In various embodiments, the oxidation and reforming catalysts are in theform of pellets or beads in a container, or coated on the walls of amonolithic structure. As used herein “monolith” or “monolithicstructure” refers to a unitary structure with one or a plurality ofchannels. In some applications, a monolithic structure, for example ahoneycomb configuration, is advantageous, because vibration, for examplein a vehicle, could cause abrasion and loss of pelleted or beadedmaterial. Further, monolithic structures typically have lower pressuredrop or back pressure with respect to the flowing exhaust stream. Amonolith is typically composed of ceramic or metal material, with theceramic or metal constructed in such a way as to form open channels fromthe inlet face, through the structure, to the outlet face and may have avariety of channel or cell sizes and shapes.

The catalyst material is typically formed into a sol or colloidaldispersion in a liquid carrier and then applied to internal surfaces ofthe monolithic metal or ceramic substrate to form a layer of catalystcoating on these internal surfaces. A review of monolithic catalyticsubstrates is provided in Heck and Farrauto, “Catalytic Air PollutionControl-Commercial Technology,” Van Nostrand Reinhold, 1995, pages19-26. A “support” or “substrate” is a material containing a catalystcomposition, which is often coated thereon. An example of a support is ahigh surface area porous material, such as a refractory oxide on which acatalytic material is deposited. A “refractory oxide” refers to amaterial that may serve as a base for incorporation of catalyticreactive species, having preferred desirable properties such as highsurface area, thermal stability at high temperature, or chemicalresistance to the reaction stream.

The cell size and shape of a monolithic structure is selected to obtainthe desired surface area, pressure drop, and heat and mass transfercoefficient required for a particular application. Such parameters arereadily ascertainable to one of skill in the art. In accordance with thepresent invention, the channel can be any shape suitable for ease ofproduction and coating, and appropriate flow of the gas stream. Forexample, for metal substrates, channels may be corrugated into straight,sinusoidal, or triangular shapes, and/or may include a herringbone orzig-zag pattern. For a ceramic substrate, the channels may be, forexample, square, triangular, or hexagonal, or any shape that can beformed by extrusion or other methods of manufacture known in the art.Channel diameters are typically in the range of about 0.001 to about 0.2inches, preferably from about 0.004 to about 0.1 inches.

In some embodiments, a metal monolithic structure with a relatively highthermal mass is used to store the heat of combustion, releasing itslowly between fuel pulses, allowing for regulation of the temperatureof the catalyst and the exhaust flowing through the catalyst at arelatively constant level (FIG. 3C). An example of this regulation isshown in FIG. 9, which depicts the average temperature of the catalystversus time as fuel is added to the gas stream in a pulsed manner asdescribed above. FIG. 9 depicts the catalyst temperature, calculated fora catalyst mass to gas flow ratio of 1250 liters per minute for acatalyst of mass of 1000 grams or 0.8 g/SLPM (standard liter per minuteof exhaust flow). The fuel is pulsed to produce a rich-lean periodicityof 0.4 Hz with an equivalence ratio of approximately 3 and typicalexhaust conditions of about 10% oxygen. “Rich-lean periodicity” refersto the reciprocal of the time in seconds of a rich plus a lean pulse.For example, a rich pulse of 0.5 seconds plus a lean pulse of 1.5seconds for a total rich plus lean time of 2 seconds would give arich-lean periodicity of ½ or 0.5 Hz. The catalyst temperatureoscillates between 655 to 681° C. (about a 26° C. variation). This is alarge oscillation and could reduce the life of the catalyst structuredue to fatiguing of the metal structures. FIG. 10 shows a series ofpoints calculated in the same way but for a wide range of catalystmasses and frequencies. Fuel pulsing above 3 Hz may be difficult toachieve and could introduce high levels of fuel penalty. Frequenciesbelow 0.4 Hz could lead to very high temperature swings for systems witha very small catalyst mass for a given flow, below 0.5 g/SLPM. Thus, thedesired rich-lean frequency range for a fuel processor according to theinvention is preferably about 0.1 to about 10 Hz, corresponding to arich-lean time period from about 10 seconds to about 0.1 second, morepreferably about 0.25 to about 3 Hz, most preferably about 0.4 to about2 Hz. In addition, the preferable range of catalyst mass is above about0.5 g/SLPM. The upper limit of catalyst mass per unit flow is determinedby desired start up speed.

In an illustrative example, a typical cylindrical catalyst structureincludes a thin 2 mil foil with 300 CPSI (cells per square inch of inletcross section area), 0.8 mm height channels, a 0.6 mg/cm² catalystcoating, external dimensions of 2 in diameter by 3 in length, and weightof about 100 g. Increasing the weight to 500 g can be achieved using thesame volume by increasing the foil thickness to 10 mil, providing aneven more stable temperature profile or the ability to operate the fuelprocessor at lower frequency of fuel pulses or higher space velocity.Due to the large difference in thermal capacity between the incoming gasstream and the catalyst substrate, a ceramic substrate may also be used.

The channel wall surfaces of a monolithic structure are coated with alayer of catalyst. The coating may be applied as a washcoat. As usedherein, “washcoat” refers to a coating applied to a substrate, such asfor example a the channel walls of a monolithic structure, typicallyconsisting of a mixture of high exposed surface area support and activecatalyst elements. (Heck and Ferrauto, supra) The high surface areasupport typically includes a porous inert oxide such as alumina orzirconia. The oxide support may include additional components active foroxidation or reforming reactions. A mixture of oxidation and reformingcatalysts is used.

As used herein, “oxidation catalyst” refers to any catalyst known in theart that is useful for the oxidation of hydrocarbons in the presence ofoxygen. A number of examples of oxidation catalysts that are useful inthe present invention are provided in U.S. Pat. No. 5,232,357.Generally, the catalytic composition includes elements of Group VI, VII,VIII, or IB of the periodic table of the elements, or combinationsthereof. Active catalytic elements include Pd, Pt, Rh, Cu, Co, Fe, Ni,Ir, Cr, and Mo. Preferably, Pd, Pt, Rh, Co, Fe, or Ni is used. Theseelements may be used separately or in combination, and either as thepure element or its oxide in actual use. A desirable property for theoxidation catalyst is that it exhibit good catalytic activity at lowtemperatures, so that the oxidation reaction may be initiated at lowtemperature. Otherwise, in an embodiment in which the gas stream is theexhaust of an automobile engine, the operation of the engine might haveto be modified to raise the exhaust temperature, which would have anegative impact on fuel economy. This property, referred to as “minimumoperating temperature,” the temperature at which the added fuel beginsto react with O₂ in the exhaust system, should be below about 250° C.,generally below about 150° C. Thus, an oxidation catalyst with lowminimum operating temperature is desirable. The oxidation catalyst maybe deposited on a support of aluminum oxide, silicon oxide, zirconiumoxide, titanium oxide, cerium oxide or a mixture or combination thereof.The catalyst may optionally include other additives or elements.Examples include cerium zirconium oxide mixtures or solid solutions,silica alumina, Ca, Ba, Si, or La stabilized alumina and other supportsknown in the art.

Use of a large loading of oxygen-storing material or catalytic metalsthat undergo oxidation/reduction cycles should be avoided, since theycould be detrimental to fuel processor performance. Such an added oxygensupply could increase the amount of oxygen available during thetransient rich portion of a pulsed fuel cycle, resulting in a highertemperature of operation and reduced fuel efficiency.

The catalyst may be prepared by impregnating Pd, Pt, or other activecatalyst material on a porous support such as alumina or zirconia. Metalloading is typically in the range of about 0.1 to about 20%, often about1 to about 10% by weight of the total washcoat material. An oxidationcatalyst for use in processing of added diesel fuel may also containcatalytic components active for steam cracking, since diesel fuel has ahigh molecular weight and a propensity to pyrolzye at high temperatures.Examples of suitable additives include basic oxides such as calciumoxide, barium oxide, other alkali or alkaline earth oxides, and rareearth oxides.

As used herein, “reforming catalyst” refers to any catalyst known in theart that is useful for production of H₂ and CO from a hydrocarbon fuel.Examples of useful reforming catalysts include Ni, Ru, Rh, Pd, and Pt.In the practice of the present invention, the reforming catalyst must bestable under the oxidizing conditions that exist under the normaloperation of a lean burn engine and must be able to respond very quicklywhen fuel is added to reform the hydrocarbon fuel to H₂ and CO.Preferably, Pt, Pd, or Rh, or a mixture thereof, is supported on aporous oxide support. An example of a typical catalyst is 1% Rh byweight supported on porous zirconium oxide. This catalyst may beprepared by dissolving rhodium trichloride in water, followed byimpregnation of this solution onto zirconium oxide with a high surfacearea, typically in the range of about 15 to about 150 m²/g. The rhodiumconcentration is typically in the range of about 0.1 to about 20% of thetotal washcoat catalyst solid, which includes the rhodium and the oxidesupport. Often, the rhodium concentration is in the range of about 0.2to about 10% of the total washcoat loading. The washcoat may be coatedonto the interior channels of a monolithic honeycomb structure at aloading or thickness of about 1 to about 50 mg/cm², often about 5 toabout 15 mg/cm² of interior geometric surface. Pd and Pt catalysts maybe prepared in a similar manner.

An oxidation catalyst and a reforming catalyst are combined in the samefuel processor. The oxidation and reforming catalysts may be on separateareas of the same monolithic structure or on separate monolithicstructures, or may be combined in the same areas of a single substrate.In one embodiment, the oxidation catalyst is separate from and upstreamfrom the reforming catalyst. In another embodiment, oxidation andreforming catalysts are combined into a washcoat to be applied to theinterior channels of a monolithic structure.

In an illustrative example, a Pd oxidation catalyst and a Rh reformingcatalyst are combined on a zirconia support to form a catalyst that hasthe oxidation activity to combust added fuel with O₂ in the exhauststream and the reforming activity to reform the remaining fuel to CO andH₂. In one embodiment, the Rh component is impregnated on the highsurface area oxide support and then calcined. Separately, the Pdcomponent is coated onto a high surface area support and calcined orfixed. The catalysts are mixed together to form a Pd/Rhoxidation/reforming catalyst. This catalyst can then be used to form acolloidal sol and then washcoated on the monolithic structure. Inanother embodiment, the monolithic structure is provided with anoxidation catalyst at the inlet and a reforming catalyst at the outlet.In a still further embodiment, the fuel processor includes two separatemonolithic structures, one with an oxidation catalyst washcoat layer atthe inlet and a second with a reforming catalyst washcoat layer at theoutlet.

In some embodiments, the oxidation catalyst and the reforming catalystcompositions include the same catalytically active component(s), forexample Pt and/or Pd. In other embodiments, the oxidation and thereforming catalyst compositions include different catalytically activecomponents.

In various embodiments of the invention, a fuel processor includes oneor more monolithic structure. In some embodiments, the fuel processorincludes one monolithic structure. In other embodiments, the fuelprocessor includes more than one monolithic structure, stacked or joinedtogether. In some embodiments, a monolithic structure includes onechannel from inlet to outlet. In other embodiments, a monolithicstructure includes a plurality of channels.

In one aspect, the invention includes a catalyst composition forproducing H₂ and CO which includes an oxidation catalyst and a reformingcatalyst. In another aspect, the invention includes a monolithicstructure onto which a catalyst composition including an oxidationcatalyst and a reforming catalyst has been coated, or onto whichseparate oxidation and reforming catalyst compositions have been coated,in the same or different areas of the substrate.

Mixing Section

In NO_(X) emission control embodiments, the lean NOx catalyst couldrequire a relatively constant concentration of reductant, e.g., H₂ andCO, to continuously reduce the NOx to N₂. For this reason, a mixer maybe provided downstream of the fuel processor to mix the reducing agentwith the exhaust flow. This mixing can occur in two separate regions,for example as shown in FIG. 4. The pulses of reductant can be mixedwith the lean pulses in region 46 and then this uniform flow can then bemixed with the main exhaust flow in region 50 to produce a relativelyuniform mixture of reductant in the exhaust stream at location 47.Alternatively, mixer 46 can be eliminated and the pulsing reductantmixed with the main exhaust flow in region 53. In one embodiment, thegas stream corresponding to several fuel injection cycles is mixed. Forexample, at a flow of 300 SLPM through fuel reformer 50 and an injectionfrequency of 1 Hz, the required mixing volume would be about 30 litersfor 2 periods (assuming a temperature of 600° C., which results in a gasexpansion of 3 fold). The higher the operating frequency, the smallerthe mixing volume needs to be to mix the H₂ pulses into a steadyconcentration. The mixing container must be catalytically inert to avoidcombustion of H₂. To further avoid this possibility, the walls of themixing container may be cooled, or the exhaust may be cooled, forexample via a heat exchanger, before entering the mixing section of theapparatus.

Alternatively, the lean NOx catalyst could require pulses of H₂ and COfor optimum performance. In this case, the mixer 46 in FIG. 4 would beeliminated and mixer 53 would be designed to mix the reductant with themain exhaust flow radially but to minimize axial mixing to retain thepulses of reductant. As discussed in Example 4 and shown in FIG. 12,some lean NOx catalysts perform better with pulses of reductants ratherthen continuous reductant concentrations. The pulses of reductantproduced in accordance with the invention may be maintained with asystem designed to minimize axial mixing, thereby maximizing the pulsesof reductant flowing through the lean NOx catalyst wherein NOx isreduced to N₂. Lean NO_(X) Catalyst

In some embodiments, the apparatus of the invention includes a leanNO_(X) catalyst, depicted as 48 in FIG. 4, for reduction of NO_(X) withthe reductant produced in the fuel processor. The lean NO_(X) catalystmust possess good selectivity for this reaction in the temperaturewindow of operation and a low activity for H₂ combustion. In oneembodiment, a catalytic structure containing the lean NO_(X) catalystcontains a combination of catalyst formulations, with a high temperatureformulation at the inlet (or outer layer for multi-layered geometry) anda low temperature formulation at the outlet (or inner layer) of thecatalytic structure.

Typical active catalytic components of the lean NO_(X) catalyst includePt, Pd, Rh, and Ir. High surface area refractory oxide supports orzeolites may be included. Typical refractory oxide supports are alumina,alumina with additives such as Si, Ca, Ba, Ti, La or other components toprovide increased thermal stability. The high surface area support maybe similar to those discussed above for the fuel processingoxidation/reforming catalysts. In addition, modifying components suchas, for example, Na, Co, Mo, K, Cs, Ba, Ce, and La, may be used toimprove the selectivity of the reaction, by reducing the oxidationactivity of the catalyst. Examples of useful lean NO_(X) catalystcompositions are discussed in U.S. Pat. No. 6,109,018.

In general it has been found that a relatively constant flow ofreductant is used to reduce NOx in the lean NOx catalyst. Surprisingly,we have found that varying concentration of H₂ and CO gives improved NOxreduction. This is shown in FIG. 12 where NOx reduction is measured fora relatively continuous concentration of H₂+CO and a pulsingconcentration of H₂+CO. This catalyst, described in Example 4, consistsof platinum and molybdenum supported on a high surface area alumina anddeposited as a washcoat onto a cordierite monolith. The pulsingconcentration of H₂+CO results in a higher level of NO_(X) conversionwith the NO_(X) conversion occurring over a much wider temperaturerange.

In some embodiments, unreacted CO may be removed by including anoxidation catalyst downstream of the lean NO_(X) catalyst, typically alow temperature oxidation catalyst. In some embodiments a low minimumoperating temperature formulation is used.

Control Strategies

A number of control strategies may be used to optimize efficiency of theemission control system of the invention. Several typical strategiesinclude:

-   -   Using one or more parameters including NOx concentration        upstream of the catalyst, NOx concentration downstream of the        catalyst, oxygen concentration in the exhaust, exhaust gas        temperature, exhaust flow rate, engine torque, engine rpm,        engine turbocharger boost, engine fuel flow, engine intake air        flow, or a variety of other engine operating parameters to        calculate the required fuel flow to the fuel injector and the        injector duty cycle.    -   Using a NOx sensor downstream of the lean NOx catalyst and then        adding fuel to the fuel processor to obtain the desired level of        NOx    -   Using a NOx sensor upstream of the lean NOx catalyst combined        with engine operating parameters to estimate the total exhaust        flow. Combining the NOx concentration with one or more of other        parameters including the exhaust flow, exhaust oxygen level and        exhaust temperature will permit an estimate of the required fuel        flow to reduce this NOx flux.    -   Using an engine map of NOx production rate or concentration        versus engine operating parameters such as but not restricted to        rpm, torque, load, or turbocharger boost. Engine parameters can        also be used to estimate the exhaust flow and therefore total        NOx flux and the required fuel flow to reduce this NOx flux. The        engine operating parameters could include rpm, engine torque,        engine turbocharger boost, engine fuel flow, engine intake air        flow, exhaust temperature or a variety of other engine operating        parameters.

The control strategy could also include the use of measured or estimatedexhaust temperature to estimate the fuel required to obtain the desiredtemperature rise. This could be combined with any of the above controlstrategies.

Adjustable parameters used to control the level of H₂ produced and thefuel processor catalyst operating temperature include flow rate of theadded hydrocarbon fuel during the rich portion of the cycle, the flowrate of the added hydrocarbon fuel during the lean portion of the cycle,the level of oxygen in the gas or exhaust stream which can be controlledor varied by engine exhaust gas recirculation (EGR) flow rate or intakeair throttle setting, the frequency of the lean and rich pulses, andduty cycle (fraction of total cycle in which fuel injection is “on”).For example, the desired flux of H₂, for example in moles per minute,FH₂, would be a function of the flux of NOx in the exhaust stream, alsofor example in moles per minute, F_(NOx), with the functionalrelationship being dependent on the performance of the lean NOx catalystas shown for example in Eqn. 1.F _(H2) =f(F _(NOx))  Eqn. 1

This functional relationship could be determined by engine tests or rigtests of the catalyst covering the expected process variables some ofwhich may be catalyst temperature, T_(cat), exhaust gas temperature,T_(exh), exhaust oxygen level, C_(O2), exhaust water concentration,C_(H2O), age of the catalyst, A_(cat), etc. The functional relationshipcould be of the form where one or more of the dependent variables areincluded in the functional relationship as shown in Eqn. 2.F _(H2) =f(F _(Nox) , T _(cat) , T _(exh) , A _(cat) , C _(O2) , C_(H2O) , A _(cat))  Eqn. 2The flux of H₂ required in the exhaust stream, defined by Eqn. 2 will bedetermined by the rate of fuel fed to the fuel processor, F_(fuel), sothe rate of fuel feed can be defined as a function of the desired H₂flux, F_(H2), as expressed in Eqn. 3.F _(fuel) =f(F _(H2))  Eqn. 3

However, since the fuel flow must be sufficient to combust the oxygenand raise the fuel processor catalyst temperature, the rate of fuel feedto the fuel processor will also be a function of the exhaust gastemperature, T_(EXh), and the desired operating temperature of the fuelprocessor catalyst in the rich zone, T_(Fp), the fraction of exhaust gasthat flows through the fuel processor, F_(FP), the oxygen concentrationin the exhaust stream, C_(O2), and the total exhaust flow rate, F_(exh),and possibly other variables. Thus, fuel flow rate could be calculatedfrom a functional relationship such as Eqn. 4 where one or several ofthe dependent variables are actually used in the functionalrelationship.F _(fuel) =f(F _(H2) , T _(Exh) , T _(FP) , F _(FP) , F _(exh) , C_(O2))  Eqn. 4

Many of the variable in Eqn. 3 are a function of the engine operatingcondition. For example, exhaust flow rate, exhaust temperature andconcentration of oxygen in the exhaust could be a function of the engineoperating conditions including engine rpm, E_(rpm), engine torque,F_(trq), engine turbocharger boost, E_(bst), engine fuel flow rate,E_(fuel), engine intake air flow rate, E_(air), engine EGR flow rate,E_(EGR), engine throttle setting, E_(thr), and possibly other variables.Thus, some of the variables in Eqn. 3 could be replaced by some of theseengine parameters and these used in the control logic to determine thedesired fuel flow rate, F_(fuel). This could be an advantageous methodof control since the engine control computer or engine control unit,ECU, may already be measuring or calculating many of these values or infact maybe setting them for correct operation of the engine. For examplecurrent engines measure rpm, engine torque, turbocharger boost and manyother variables and control or set such variables as EGR flow rate, fuelflow rate, turbocharger boost, etc.

In an alternative control strategy, the engine could be mapped over theentire engine operating range of rpm and torque and include suchvariables as inlet air temperature or ambient temperature. Over theseoperating ranges, the exhaust composition including oxygenconcentration, NO_(X) concentration, exhaust flow rate, exhausttemperature and may other important variables could be determined and amap generated that would specify the value of fuel flow required. Thismap or look up table could then be used to determine the required fuelflow rate at any engine operating condition. This fuel flow rate wouldthen be used to set the fuel flow rate during the rich pulse and theduty cycle. Alternatively, a combination of these strategies could beused. For example, engine operating parameters could be used todetermine the NOx concentration in the exhaust stream using a fast lookup table. Then, a simple functional relationship could be used tocalculate the fuel feed rate based on some engine parameters such asrpm, torque, EGR flow rate or setting, inlet throttle setting andturbocharger boost pressure or some subset of these parameters.

In most applications, the engine will operate in a transient manner withthe engine rpm and torque varying with time. Control of the fuelinjection rate in these cases will be similar or may require theaddition of a delay period based on the rate of change of certain engineoperating parameters. For example, as engine rpm is increased, theexhaust flow rate will increase and therefore the fuel flow required tomaintain the fuel processor catalyst temperature will increase. However,some delay in the fuel flow may be required since the exhaust flow rateincrease may lag behind the actual rpm increase. Similarly, as torqueincreases, NO_(X) level and exhaust flow may increase and fuel flow tothe fuel processor may need to be increased, but both of theseparameters may be delayed compared to the steady state engine operatingparameters. The fuel flow to the fuel processor catalyst may need to beadjusted so that temperature rise through the fuel processor catalystand the amount of reductant produced effectively match the actual fuelprocessor catalyst inlet temperature and the level of NO_(X) enteringthe lean NO_(X) catalyst.

The following examples are intended to illustrate, but not limit theinvention.

EXAMPLES Example 1 Catalyst Preparation

A monolithic structure was prepared as described in U.S. Pat. No.5,259,754.

Palladium nitrate was diluted in deionized water at a concentration ofabout 0.18 g Pd/ml and then zirconium oxide powder having a surface areaof about 75 m²/g was added with stirring. The mixture was thenevaporated to dryness and the resulting powder calcined in air at 700°C. for 10 hours. The final palladium concentration in the final catalystpowder was 5.5% by weight. The Pd/ZrO₂ catalyst was slurried with waterand 10% by weight of a 20% zirconium acetate solution to form a slurryof about 30% solids. The Pd concentration in the solid oxide was 5%.

Rhodium trichloride was dissolved in deionized water at a concentrationof about 0.04 g Rh/ml and then zirconium oxide powder with a surfacearea of about 75 m²/g was added with stirring. While stirring themixture, a solution of 20% ammonium hydroxide in water was added to a pHof 8. the mixture was then evaporated to dryness and the resultingpowder calcined in air at 700° C. for 10 hours. The final rhodiumconcentration in the final catalyst powder was 1.1% by weight. TheRh/ZrO₂ catalyst was slurried with water and 10% by weight of a 20%zirconium acetate solution to form a slurry of about 30% solids. The Rhconcentration in the solid oxide was 1%.

A strip of River Lite 20-5Sr from Kawasaki Steel Company with athickness of 0.050 mm, width of 75 mm, and length of 3 m was corrugatedto form V-shaped channels in a herringbone pattern. The channels wereapproximately 10 mm in width and 1.46 mm in height with an angle betweenthe herringbone sections of 15 degrees. The corrugated foil was heatedin air at 900° C. for 10 hours to form a layer of aluminum oxide on thesurface. The Pd/ZrO₂ slurry was sprayed onto the foil to form a catalystcoated layer 25 mm wide along the 3 m length on both sides of thecorrugated foil coating the same section of the foil on both sides. Thecatalyst layer had a loading of 6 mg/cm² of the geometric foil metalsurface. The Rh/ZrO₂ slurry was then sprayed onto the remaining uncoatedmetal surface on both sides of the foil to a loading of approximately 6mg/cm². The coated catalyst was then calcined in air at 700° C. for anadditional 10 hours. The foil was then folded in half and rolled to forma non-nesting spiral roll with open longitudinal channels. The finalcatalyst had a diameter of 50 mm and contained about 13 g of catalystwashcoat.

Example 2 Catalytic Conversion of Fuel Injected at Varying Frequenciesto H₂

The catalyst from Example 1 was placed in a flow reactor containing agas supply and mass flowmeters for air, N₂ and He as a mass spectrometerquantitative tracer, an electric heater, an air assist sprayer forwater, and a pressurized fuel sprayer for diesel fuel injection(Mitsubishi MR560553). The catalyst was located in a 50 mm diameterinsulated section with thermocouple temperature sensors upstream anddownstream of the catalyst. A sampling probe for a mass spectrometer waslocated about 50 cm downstream of the catalyst outlet.

The flow of air, nitrogen, and water was adjusted to a total gas flowrate of 600 SLPM, forming a final composition of 5% H₂O, 7% O₂, 0.3% He,and the balance N₂. This mixture was then heated to 270° C. using theelectric heater. Diesel fuel was injected in pulses of varyingfrequency, as shown in FIG. 6. The mass spectrometer sampling rate was 2Hz, and the injection frequency varied between 0.4 and 1 Hz. FIG. 6Adepicts the O₂ and H₂ concentrations as a function of time and fuelpulse frequency, converted to concentration units of percent of volume.The sampling point for this location was just downstream of the fuelprocessor unit equivalent to sampling at location 7 in FIG. 2. Duringpulses of fuel injection, all of the O₂ present during the fuel “on”phase is consumed and H₂ is produced.

FIG. 6B depicts the temperature of the gas at the outlet of thecatalyst, measured by three thermocouples placed immediately downstreamof the catalyst at different locations across the outlet face. Previouswork has shown that the gas temperature responds very rapidly to anychange in the catalyst temperature. The temperature of the gas streamexiting the fuel processor is very constant showing no oscillations fromthe pulsing fuel and thus demonstrating the inventive concept that thethermal mass of the catalyst can be used to dampen the oscillations frompulses of fuel. At 0.4 Hz, the temperature is approximately 600° C. andrises to about 725° C. as the frequency is increased to about 1 Hz. Thisrise in temperature is due to reaction of increasing amounts of O₂ withthe fuel resulting in a larger heat release.

Example 3 Effect of Mixing to Provide a Continuous Stream of H₂ and CO

An experiment similar to Example 2 was performed, but using 22 gcatalyst with an 8 mm channel foil height, at a flow rate of 350 SLPM.Fuel pulses were at 0.5 Hz. After exiting the fuel processor catalyst,the resulting gas stream then entered a 38 liter stainless steel mixingchamber, to mix the resulting products. A mass spectrometer was used tofollow the composition of the gas stream just downstream of the fuelprocessor, equivalent to location 7 in FIG. 2 and just downstream of themixing volume, equivalent to location 10 in FIG. 2. The H₂ concentrationjust downstream of the fuel processor is shown in FIG. 7A and the H₂concentration downstream of the mixing volume is shown in FIG. 7B. Thesedata show that such a mixing volume is sufficient to mix out the pulsingH₂ and provide a stream of gas with a relatively constant level of H₂.It should be noted that CO was not monitored in these tests but the COconcentration is expected to show the same behavior as the H₂

Example 4 Effect of Pulsing and Continuous Reductant on NOx Conversion

A lean NOx catalyst was prepared as follows. A gamma alumina powder witha surface area of about 250 m² μg was impregnated with ammoniumheptamolybdate dissolved in de-ionized water and the powder dried andcalcined at 600° C. to form a final Mo loading on the alumina of 10% byweight. This powder was then impregnated with platinum nitrate solution,dried and calcined at 600° C. to form a final Pt loading of 0.5%. Thisfinal solid was then milled in a ball mill with water to form a sol ofabout 30% solids by weight and this sol coated onto a cordieritemonolith and dried and calcined at 600° C. to form a final structurethat contained about 15% of the sol as a washcoat. A 25 mm diameter by75 mm cylinder of this catalyst was placed in a test system that had aflowing gas stream at 25 liters per minute containing 10% O₂, 8% CO₂, 5%H₂O, 10 ppm SO₂ and 600 ppm NO. Into this stream was added H₂ and CO toform a concentration in the flowing gas stream of 6000 ppm of H₂ and3000 ppm of CO. This H₂ and CO was added either as a continuous flow orinjected at about 0.3 seconds every 1 second. The NOx level downstreamof the lean NOx catalyst was monitored and as the lean NOx catalysttemperature was varied from 150 to 425° C., the NOx conversion curvesshown in FIG. 12 were obtained. Pulsing or varying levels of H₂ and COshow a substantially improved NOx conversion.

Although the foregoing invention has been described in some detail byway of illustration and examples for purposes of clarity ofunderstanding, it will be apparent to those skilled in the art thatcertain changes and modifications may be practiced without departingfrom the spirit and scope of the invention. Therefore, the descriptionshould not be construed as limiting the scope of the invention, which isdelineated by the appended claims.

All publications, patents and patent applications cited herein arehereby incorporated by reference for all purposes and to the same extentas if each individual publication, patent or patent application werespecifically and individually indicated to be so incorporated byreference.

1-65. (canceled)
 66. A method of forming reformate from diesel enginefuel in a diesel engine exhaust stream, comprising: channeling theexhaust through an exhaust system that fixedly divides the exhaust intoat least a first exhaust stream and a second exhaust stream, the streamsfollowing, respectively, a first fixed flow path and a second fixed flowpath, wherein a fuel reformer is configured in the first flow path andthe second flow path bypasses the fuel reformer; injecting the dieselfuel into the first exhaust stream within the first flow path to form anoverall rich composition; reforming a portion of the injected fuel inthe fuel reformer to form H₂ and CO; and causing the first and secondexhaust stream to recombine downstream from the fuel reformer.
 67. Themethod of claim 66, wherein the recombined stream is channeled through alean NOx catalyst that reduces NOx contained in the recombined exhauststream under lean conditions using H₂ and CO produced by the fuelreformer as a reductant
 68. The method of claim 66, wherein the secondflow path is essentially empty of exhaust treatment devices.
 69. Themethod of claim 66, wherein the flow rate and oxygen concentration ofthe exhaust entering the first flow path are uncontrolled.
 70. Themethod of claim 66, wherein the reformer produces H₂ and CO at least inpart through steam reforming reactions.
 71. The method of claim 70,wherein at least a portion of the injected fuel is oxidized within thefirst flow path to provide energy that heats the reformer to reach ormaintain steam reforming temperatures.
 72. The method of claim 70,wherein a portion of the injected fuel is oxidized within the first flowpath at a location upstream from where H₂ and CO are formed, therebyconsuming substantially all of the oxygen contained in the exhaustfollowing the first flow path.
 73. The method of claim 66, whereinessentially all of the oxygen in the exhaust is consumed oxidizinginjected fuel over an oxidation catalyst upstream from where the exhaustis divided between the first and second flow paths.
 74. The method ofclaim 66, wherein from about 5 to about 25% of the exhaust iscontinuously diverted through the first low path.
 75. The method ofclaim 66, wherein the fuel injection is set based on the exhaust oxygenconcentration to achieve a target reductant amount in the first flowpath after oxygen in the first flow bath has been substantiallyconsumed.
 76. The method of claim 75, wherein the target reductantamount is independent of the exhaust oxygen concentration.
 77. Themethod of claim 66, wherein the first and second flow paths arespatially contiguous.
 78. A power generation system comprising: anengine operative to produce an exhaust stream; an exhaust systemconfigured to receive the exhaust and fixedly divide the exhaust streaminto first and second flow paths and to recombine the exhaust downstreamfrom the first and second flow paths; and a fuel reformer configuredwithin the first flow path.
 79. The power generation system of claim 78,further comprising a lean NOx catalyst within the exhaust systemdownstream from where the exhaust from the first and second flow pathsrecombines.
 80. The power generation system of claim 78, wherein thesecond flow path is essentially empty of exhaust treatment devices. 81.The power generation system of claim 78, wherein the fuel reformer isadapted to produce H₂ and CO at least in part through steam reformingreactions.
 82. The power generation system of claim 78, wherein thefirst flow path comprises an oxidation catalyst configured to combustinjected fuel to provide energy to heat the fuel reformer to reach ormaintain steam reforming temperatures.
 83. The power generation systemof claim 78, wherein the first flow path comprises an oxidation catalystconfigured upstream from the fuel reforming catalyst.
 84. The powergeneration system of claim 78, wherein the exhaust system is configuredto fixedly direct from about 3 to about 30% of the exhaust through thefirst low path.
 85. The power generation system of claim 78, furthercomprising a controller that controls the fuel injection into the firstflow path based on an amount of oxygen in the exhaust entering the firstflow path, wherein the controller does not control the exhaust flow ratethrough the first flow path.
 86. The power generation system of claim78, wherein the first and second flow paths are spatially contiguous.87. The power generation system of claim 78, wherein the first andsecond flow paths are defined by a monolith catalyst occupying only aportion of a channel, the first flow path passing through the monolithcatalyst and the second flow path lying outside the monolith catalyst.88. The power generation system of claim 78, further comprising anoxidation catalyst configured within the exhaust system upstream fromthe first and second flow paths.